Gas turbine engine compressors include an engine casing and a number of rotating blades disposed within the casing. As the blades rotate within the casing, there is a certain amount of clearance between the tip of the blades and the surface of the engine casing in order to prevent the tips of the rotating blades from contacting the engine casing and causing damage to the casing and the blade tips. In aircraft engines, it is desirable to minimize the amount of clearance between the tip of the compressor blade and the engine casing in order to maximize the engine's efficiency. From the standpoint of performance, the blade tip clearance should ideally be zero. However, from a practical standpoint, some tip clearance is necessary to avoid blade tip rub against the casing. The blade tip clearance has a significant effect on the compressor performance including aerodynamic efficiency, pressure ratio, and compressor stall margin. In the past, abradable seals have been utilized between the engine casing and the tip of the rotating compressor blade in order to minimize gap clearance to about 0.025″, as measured between the tip of the rotating blade and the abradable seal.
A conventional abradable seal is illustrated in FIG. 1. With such seals, a rigid shroud casing 1 usually includes an inner surface having either a felt metal or a soft coating 2, typically sprayed nickel containing graphite. Both types of abradable seals are weak enough to abrade away when contacted by a high speed rotating blade 3, without wearing or damaging the blade tip during short-duration rubs, such as rubs which may occur during compressor stalls and hard landings. The designed clearance, “c”, of the rigid abradable casing is large enough to avoid frequent rubs. However, during a short-duration rub, the clearance is further enlarged, permanently reducing the compressor performance, such as its efficiency and stall margin. Beyond a certain tip clearance enlargement and performance loss resulting from accumulated rub damage, the engine is removed from service and overhauled.
Conventional rigid shroud casings degrade compressor performance as the abradable seal wears away, and also increases the maintenance cost due to frequent engine overhaul and lost service. In order to further reduce the gap clearance and improve performance, a compliant shroud has been developed by Compressor Aero Research Laboratories (CARL), Wright Laboratories WPAFB, Dayton, Ohio, which incorporates staged conventional brush seals, as shown in FIG. 2. In this design, a number of conventional bristle packs 4 are attached to the rotor 5, and are further welded between a rigid front plate 6 and a rigid back plate 7. The bristles are inclined in the direction of rotation, as is conventional with brush seals, and the gap clearance between the tip of the bristles and the tip of the blade is reduced significantly, to about 0.005″. In the event of a blade rub, the bristles deflect elastically and return to their original configuration, thus not enlarging gap clearance. Although the gap clearance is reduced, the clearance of the gap is not continuous since a continuous compliant brush surface is not present, due to the incorporation of front and back plates. The lack of a continuous surface can result in the creation of vortices within the region, thus lowering the compressor efficiency. Another important drawback of the CARL design is that the use of redundant front and back plates significantly increased the weight and cost of the system, which is particularly undesirable in aerospace applications where weight and cost reduction is important. In addition, welding the bristles to the front and back plates can also lead to increased cost, both initially and later, during repairs.
A hybrid brush shroud design has also been proposed by CARL, as illustrated in FIG. 3. In this design, a compliant brush casing containing multiple bristles 8 in approximately the front 30% of the chord length is abutted by a rigid shroud ring 9. Using fluid mechanical modeling, it has been demonstrated that the blade tip clearance is critical only in the front section. The tip clearance is significantly reduced in the front section containing the bristle packs. The rear section, containing the rigid shroud ring has an increased tip clearance, in order to prevent blade tip rub. This hybrid structure will reduce the fraction of the blade chord covered with expensive brush shroud, resulting in a lower cost. The brush shroud structure is placed only where it is needed, not the entire chord length. While FIG. 3 describes a concept, no design and fabrication methods have been proposed by CARL. In addition, if the rigid shroud ring is contacted by the rotor blade, it can damage both the blade and the shroud ring.
Therefore, there is a need in the art a compliant shroud casing which has a good resistance to damage caused by short-duration rotor rubs so as to allow for increased performance overall and less frequent engine repairs due to blade rub, and which is light-weight and cost effective to manufacture and replace, if needed.